Pneumatic shaped solid tires are known in the prior art and are used to replace pneumatic tires in many industrial applications, such as forklifts, baggage carts, and catering vehicles. Solid tires offer the advantages of increased load-bearing capabilities and elimination of the possibility of dangerous blow-outs and costly flat tires.
A typical prior art pneumatic shaped solid tire is illustrated in partial cross-sectional view in FIG. 1 and indicated generally at 10. The outer portion 12 of the tire 10 is a tread rubber compound which is compounded to give wear and grip to the tire, and is maximized for high abrasion, high tear resistance and strength, and low rolling resistance. Typical prior art treads 12 are formed from black compound, comprising a mixture of natural rubber (which can be contain a ratio of synthetic rubber), carbon black, a cure system, an aging system, and a plasticity system for the vulcanization process. Other tread compounds are known in the prior art. For example, a white compound is sometimes used for the tread 12, which is the same as the black compound but has white silica substituted for the carbon black. Beneath the tread compound 12 is a hard base rubber layer 14 which is used to grip the rim of the wheel (not shown) upon which the tire 10 is mounted and to support the tire 10 upon this rim. A typical prior art material used for the hard base rubber layer 14 is known as cording compound, which comprises synthetic rubbers (and possibly a ratio of natural rubber), long-length fibers, carbon black, a cure system, and an aging system for the vulcanization process. The cording compound is compounded for hardness and high modulus. The tire 10 may also have a bead 16 on each side of the tire 10, located near the inside perimeter of the tire 10. The bead 16 provides strength to the tire at the intersection with the lip of the rim upon which the tire 10 is mounted. The beads 16 may be pneumatic beads which may be assembled with ply fabric, for example. Alternatively, the beads 16 may comprise steel round wire which may be assembled in a cage.
Because the tread compound 12 is somewhat hard in order to provide high wear resistance and low rolling resistance, and because the layer 14 is also hard in order to provide structural rigidity to the tire at its mounting location, it has been found that the tire 10 may not provide a soft enough ride in some industrial applications, such as with forklifts carrying fragile cargo. The vibration transmitted through the tire 10, to the forklift and to the cargo, caused by the tire 10 rolling over surface irregularities, is unacceptable for some applications. Thus, a second embodiment prior art tire is illustrated in FIG. 2 which alleviates some of these problems, and is indicated generally at 20. The tire 20 includes the tread compound layer 12, the hard base rubber layer 14, and the beads 16 of the tire 10. However, a soft cushion rubber layer 22 is formed between the tread compound 12 and the hard base rubber layer 14. The cushion rubber layer 22 is formed from a softer rubber compound in order to absorb a portion of the shocks and vibrations produced by the tire 20 rolling over irregular surfaces. This translates into a much softer and less jarring ride for the piece of equipment riding upon the tires 20.
While prior art solid tire designs such as the tires 10 and 20 illustrated in FIGS. 1-2 provide adequate performance in industrial settings, they suffer from the problem that a majority of the load weight is supported by the relatively small portion of the tire which lies directly under the wheel at any given time. In other words, the portions of the tire to either side of the wheel and above the wheel (which comprises a majority of the tire) do not support any appreciable portion of the load weight. By concentrating the load weight on a small section of the tire at any given time, the tire is forced to endure much higher stresses than would be the case if the load were evenly distributed around the entire tire. Such increased effective loading results in faster tire wear than would be the case with distributed loading.
Furthermore, while prior art solid tire designs, such as the tires 10 and 20 illustrated in FIGS. 1-2, provide adequate performance in industrial settings, they suffer from the problem that the tire must be scrapped after the relatively thin thread portion has been worn away. As the remaining portion of the tire that is scrapped comprises a majority of the cost of the tire, this situation is economically undesirable.
There is therefore a need for a solid pneumatic-shape tire system in which the entire tire does not need to be scrapped after the tread has worn out. There is also a need for a solid tire design which provides for more effective distribution of the load weight throughout the entire tire, thereby providing higher load capacity for the tire and a more effective tire system. The present invention is directed toward meeting these needs.